At the start of the first campaign in Left 4 Dead 2, the playable characters have just met each other and do not even know each other's names. In Left 4 Dead, it is never explained how they met up with each other, or how long they have been together (although judging by the timing given by the intro cutscene and The Sacrifice comic, it is less than 12 days by the start of the first campaign). In addition, The Sacrifice comic presents the hypothesis that the Survivors are carriers of the Infection―and as such are a positive danger to any uninfected humans they encounter.
Although the playable characters are immune, a comment by Bill suggests that they could become an Infected at any point. The term, however, is commonly used to refer to the eight playable characters from the Left 4 Dead game series. The Survivors are people in the world who either have not yet contracted the Infection (by virtue of natural immunity) or are carriers of the disease but who display none of its symptoms―and who may thus falsely believe themselves to be immune.
